Residual perceptions are common in everyday life. If you look at the sun briefly and then look away, the image of the sun remains in your eye for a while. In the same way, Aristotle asserts that when the external object of a perception has been removed, there still remains an impression which itself is an object of perception. While we sleep, these impressions are re-created in dreams.
Sensory impressions present themselves when an individual is awake and asleep, but the senses work with the brain during the day to keep reality in perspective. At night there is no balance between impressions because the senses are not available, so dreams produce wild and obscure sensations.